Many virus particles have an isoelectric point (pI) of from 3 to 6, and such viruses are negatively charged in neutral pH range. When such viruses are located in highly pure water, the viruses may be adsorbed onto an appropriate substrate by electrostatic interaction between the virus and the substrate.
Japanese Patent Application Laid-Open No. 3(1991)-123630 discloses a virus removing material comprising a substrate such as a porous membrane having polyvinyl-pyridinium structure thereon. This porous material, however, suffered from non-specific adsorption of proteins, and when the material was used for virus removal from fluids such as blood and plasma, which usually contain proteins at a high concentration, it failed to selectively adsorb the virus.
Another filter for removing virus from body fluids and other protein solutions is disclosed in Japanese Patent Application Laid-Open No. 2(1990)-167232. This filter comprises a regenerated cellulose membrane having a pore size smaller than the HIV particle, and the passage of the HIV particles upon filtration through the membrane is thereby inhibited. Such small pore size, however, also resulted in low filtration rate as well as frequent clogging. In addition, there has been reported that the pathogenicity of the HIV is found not only in the HIV particle, but also in the envelope proteins such as gp120 and gp160 (see, for example, Microbiological Reviews, March 1993, pages 183-289, "Pathogenesis of HIV infection".) The pore size of this filter is not small enough to capture the gp120.
Another virus removing material is disclosed in WO89/01813, that utilizes biological affinity for the removal of the virus from the blood or plasma. This material comprises a substrate having immobilized thereon a virus-binding site (receptor) of a cell, that is found on the surface of the cell. Production of such material, however, involves complicated steps of isolation of the receptor from the cell and its purification followed by immobilization of the purified receptor onto the substrate. Use of such receptor of biological origin also resulted in an increased cost and some risk of unstable performance and alteration in the quality after storage.
Japanese Patent Application 2(1990)-36878 (U.S. Pat. No. 5,041,079) discloses a material for removing HIV and its related substances comprising a solid substance with a weakly acidic or weakly alkaline surface at a pH of from 2.5 to 6.9 or from 7.4 to 10.5. This material may typically have --COOH or --SO.sub.3 H on its surface, and there is clearly stated that "substances whose surface pH is in the range 7.0 to 7.3 (e.g. nonpolar polypropylene, quartz, and cation-exchange resins of --SO.sub.3 Na type) are unable to absorb HIV and/or its related compounds" (col. 2, lines 29 to 33 in U.S. Pat. No. 5,041,079). Furthermore, it has been confirmed that the body fluid would undergo an alteration in its pH upon contact with such an HIV removing material, and use of such material is not necessarily favorable for blood and other body fluids since such alteration in the pH may result in the denaturing of the proteins.